Pipes of polymer material are frequently used for various purposes, such as fluid transport, i.e. transport of gases or liquids. The fluid may be pressurised such as when transporting natural gas or tap water, or not pressurised such as when transporting sewage (waste-water), drainage (land and road drainage), for storm water applications or for indoor soil and waste. More-over, the transported fluid may have varying temperature, usually within the temperature range from about 0° C. to about 50° C. Pressureless (non-pressure) pipes may also be used for cable and pipe protection.
Such pressureless pipes are herein also referred to interchangeably as sewage pipes and pressureless sewage pipes.
The term “pipe” as used herein is meant to comprise pipes in a broader sense, as well as supplementary parts like fittings, valves, chambers and all parts which are commonly necessary for e.g. a sewage piping system.
Pipes according to the present invention comprise single or multilayer pipes, where for example one or more of the layers is a metal layer and which may include an adhesive layer. Structural-wall pipes, such as corrugated pipes, double-wall pipes with or without hollow sections, are also comprised by the present invention.
Different requirements are imposed on pipes for the transport of pressurised fluids (so-called pressure pipes) and pipes for the transport of non-pressurised fluids such as sewage (so-called pressureless pipes). While pressure pipes must be able to withstand an internal positive pressure, i.e. a pressure inside the pipe that is higher than the pressure outside the pipe, pressureless pipes do not have to withstand any internal positive pressure, but are instead required to withstand an external positive pressure, i.e. the pressure outside the pipe is higher than the pressure inside the pipe. This higher outside pressure may be due to the earth load on the pipe when submerged in the soil, the groundwater pressure, traffic load, or clamping forces in indoor applications. There is thus a clear distinction between pressure pipes on the one hand and pressureless pipes on the other hand. As mentioned above, the present invention relates to pressureless pipes.
With regard to prior art relating to propylene polymers and pipes of polypropylene the following references may be mentioned.
EP 1 028 985 relates to nucleated propylene homo- and copolymers e.g. for tubes, pipes and fittings. The propylene polymer is prepared by polymerization in the presence of a catalyst system primarily transesterified with a phtalic acid ester—a lower alcohol pair and comprising a donor and a polymerized vinyl compound such as vinyl cyclohexane (VCH) as a nucleating agent.
WO 97/13790 relates to a process of making propylene polymers or copolymers in propylene medium at super-critical temperature and pressure conditions.
EP 0 808 870 relates to a high molecular weight reactor blend of polypropylene and an ethylene-propylene copolymer with an ethylene content of 0.1-2% by weight. The blend has a MFR (5/230) of at most 5 dg/min and a broad molecular weight distribution (Mw/Mn) of 6-20.
EP 0 791 609 relates to a high molecular weight polypropylene with broad molecular weight distribution. The polypropylene is a high molecular weight copolymer of ethylene and propylene with 1-10% of ethylene and a MFR (5/230) of less than 5 dg/min and a Mw/Mn of 6-20.
WO 99/35430 (=U.S. Pat. No. 6,433,087) relates to a heterophasic propylene copolymer with a propylene homopolymer matrix and an ethylene-propylene copolymer as a dispersed elastomeric component. The heterophasic propylene copolymer has a tensile modulus of 1300-2300 N/mm2 and an impact strength at 23° C. of 60-110 kJ/m2.
EP 0 877 039 relates to a reactor blend of a propylene homopolymer and an ethylene-propylene copolymer which may be used as moulding composition for automotive parts such as bumpers, instrument panels and the like. The reactor blend has an ethylene content of 0.5-25% by weight, a MFR (5/30) of at least 5 dg/min, and the copolymer comprises 13-40% by weight of ethylene repeating units.
Pressureless pipes such as sewage pipes are made in a variety of dimensions from about 0.1 to about 3 m diameter and of a variety of materials such as ceramics (vitrified clay mainly), concrete, polyvinyl chloride (PVC), polyethylene (PE), and polypropylene (PP). While ceramics and concrete are low-cost materials, they are unfortunately heavy and brittle. There has therefore been a trend during recent years to replace sewage pipes of ceramics or concrete with pipes of polymer materials such as PVC, PE or PP. While PVC costs less than PP per unit weight, PP has advantages over PVC in other respects by having a lower density and thus a lower weight per metre pipe, having superior high and low temperature properties, and being weldable.
Sewage pipes of PP must show sufficient stiffness to withstand the earth load without any help from internal pressure. The stiffness of the pipe is derived primarily from the pipe material and as a measure of the stiffness may be taken the elasticity modulus (or modulus for short) of the pipe material. The higher the modulus of the pipe material, the stiffer the pipe will be. The stiffness of the pipe may be further enhanced by the design of the pipe wall, e.g. by corrugating the pipe.
Further, pressureless pipes are often exposed to high as well as low temperatures. They must therefore be durable within a wide range of temperatures which means that they should display a high impact strength, particularly at low temperatures.
The pipe should not be brittle, because if it is too brittle the pipe will fail due to brittle cracking. A measure of the brittleness of the pipe is its resistance to slow crack growth. The higher the resistance to slow crack growth, the better.
When a material with a higher modulus is used a thinner pipe wall may be used while obtaining the same or higher (ring) stiffness as a lower modulus pipe with a thicker pipe wall.
Thinner pipe walls are more sensitive to cracks since any damage or notch on the pipe surface will propagate easier through the pipe wall. Structured-wall pipes (corrugated, ribbed, twin-wall pipes, etc.) are most sensitive to cracks and the slow crack growth properties of the material since the structured pipe design often consists of thin sections.
Structured-wall pipes includes for example single-layer corrugated pipes, ribbed pipes, twin-wall pipes with hollow sections, multilayer pipes with or without hollow sections or foamed layers, and spirally wound pipes with or without hollow sections with smooth or corrugated pipe design.
Basically, pipes with thin sections, either smooth solid-wall pipes of smaller diameters or structured-wall pipes with thin sections are more sensitive to cracks. Due to the high and 3-dimensional structure of structured-wall pipes also the stresses are locally higher when subject to external load conditions compared to smooth solid-wall pipes, i.e. larger sensitivity to cracks.
When using materials with higher stiffness, the stress in the pipe wall will be higher when buried under ground due to the higher load bearing capability of the pipe and the constant deflection condition.
The stiffness and brittleness are two contradictory properties. Accordingly, the stiffer a pipe is, the more brittle it will normally be. Thus, a high modulus is usually accompanied by a low resistance to slow crack growth.
In view hereof there is a need for an improved pressureless pipe of a polymer material that combines a high stiffness, a high impact strength, and preferably also a low brittleness, i.e. the pipe should have a high modulus, be durable especially at low temperatures, and have a high resistance to slow crack growth.